Whoa, Now why are you taking the other 6770 out? Not understanding the other posts on why your taking the card out. 2 6770's will perform better then a single 7870, but Not all games will support it, That is why AMD (ATI) now made drivers where you can pick and choose which crossfire profile to work with that game. For ex:
My 5850's in Dayz doesn't work well, Looking at MSI Afterburner, My 2nd 5850 isn't even being touched, So I leave the game and open CCC and gaming tab scroll all the way down and select from the drop down "Use AMD Pre-defined Profile" and type in BF3 and select BF3.exe from the list and then apply, start up DayZ my performance nearly doubled in the game to 50fps looking in the same spot in the armory by myself. Thats with a 970 mobo x16/x4
You will have to play with the settings to get the performance just right. I do not get any micro stutter, but I have on some games, but finding a different profile to run with the game cured that. SLI is no better and have to do the same thing in the Nvidia Control Panael for some games. Yes a single 7850/70 will be easy to mess with, less power, don't have to worry about profiles.
Also if just plugged in the other 6770 and think you will get a performance boost, you wont, You will need to re install the driver for it to work properly.